IX.] CAUSES OF STERILITY^ 243 



generally caused in this country by the absence of 

 calcium carbonate from the soil. When this happens 

 on light sandy land it will become evident by the 

 tendency of black mild humus to accumulate, by the 

 paucity of leguminous plants in the herbage, and by the 

 prevalence of fungoid diseases like " finger-and-toe." 

 On strong lands, and when accompanied by water- 

 logging, black acid peat accumulates : the soil shows 

 an acid reaction, oxide of iron forms below the surface, 

 and the soil water contains soluble iron salts, as is seen 

 by the iridescent scum which spreads over any water 

 standing in the ditches. 



Another source of sterility is the presence of un- 

 oxidised iron salts in the soil : many clay subsoils are 

 coloured dark blue or green by double ferrous silicates 

 like glauconite, or by finely disseminated iron pyrites. 

 Until these materials become oxidised to ferric hydrate, 

 the soil remains sterile: particularly is this the case 

 with iron pyrites, which in the form of marcasite easily 

 oxidises to yield both ferrous sulphate and sulphuric 

 acid. Voelcker has recorded three instances of soil 

 sterile through these causes : one was land reclaimed 

 from the bed of the Haarlem Lake, which contained 071 

 per cent, of iron pyrites and 0-74 per cent, of ferrous 

 sulphate, as well as some insoluble basic sulphate of 

 iron. Another example of land reclaimed from the 

 sea contained 0-78 per cent, of pyrites and 1-39 per 

 cent, of ferrous sulphate. Cultivation with a free use 

 of lime and chalk is the best means of ameliorating 

 such soils, which always show an acid reaction. 



Kearney and Cameron in America have shown that 

 salts of magnesia possess, even in solutions of great 

 dilution, a toxic action upon plant roots, which is much 

 diminished if calcium salts be present at the same time. 

 Loew at the same time has indicated that a comparative 



